1. Field of the Invention
The present invention relates to the field of eye observation systems used in the ophthalmic fields of corneal topography, ophthalmometry, keratometry and tonometry, and particularly to apparatus used in placido-based eye observation systems for positioning the placido at the optimum distance from the eye along a viewing axis.
2. Description of Related Art
Eye observation systems sometimes feature placidos, also known as placido members, for projecting a special light pattern onto the eye that is observed. The eye is observed along a viewing axis through a hole in the center of the placido member. The light pattern is projected from a special placido surface in the front side of the member. The placido surface is typically either flat or three dimensional, e.g. a cone. The surface consists of a series of concentric rings of alternating dark and bright colors. The bright rings, which are also known as mires, are typically translucent and illuminated from the rear. The surface thus projects images of the mires onto the eye. The mire pattern is reflected on the cornea of the eye, and is thus observed together with the eye.
The eye observation system is typically designed to observe the eye when the cornea is at an optimum distance from the placido surface. While that distance is fixed, the mechanism needed to achieve it must be adjustable, as individual faces vary. This is a well known problem in the prior art. Additional information is found in U.S. Pat. No. 4,881,807 to Luce et al., and in PCT Application No. PCT/US95/13993 published May 9, 1996 as publication No. WO 96/13199, the contents of which documents are incorporated herein by reference.